One type of system for locking a closed overhead garage door, includes a latch mounted on the garage door and a solenoid mounted on the door frame and having a plunger that passes through a hole in the latch. A system of this type is described in U.S. Pat. No. 4,254,582 by McGee, for Electrically Actuated Overhead Garage Door Opener Assembly. A clearance must be left between the side of a garage door and the door frame, and vandals may try to open or break the lock by tools inserted through the clearance space. One technique used by vandals is to insert a bolt or other member so its end lies against the plunger, and then hit the bolt with a hammer, in order to break the solenoid loose from its mounting. There is a danger that the solenoid will break free, because it must be held sufficiently rigid so the plunger is maintained in alignment with holes in the lock box and latch for normal operation. A load in the opposite direction can occur if the garage door closes when the plunger is extended.
Another problem that must be overcome with such latching systems is to provide for manual opening of the lock in the event of a power failure that prevents energizing of the solenoid to withdraw the plunger. Any arrangement which permits an authorized user to withdraw the plunger in the event of a power failure, should not make the lock vulnerable to withdrawal of the plunger by a vandal. A garage lock system which resisted tampering by vandals or damage from inadvertent door closing, while assuring secure locking and unlocking under normal conditions, and while permitting authorized users to unlock the lock in the event of a power failure, would be of considerable value.